Since the complex sequence of electrophysiological events which incite life-threatening ventricular arrhythmias (VA) are poorly understood, most patients at risk go unrecognized until after they have experienced (and survived) a VA. Consequently, sudden cardiac death remains a major unresolved public health problem. Recently, there has been extraordinary interest in the possible roles of cardiac repolarization in the mechanism of VA. In particular, subtle beat-to-beat alternation of ECG T wave shape (i.e. T-wave alternans), which is a significant marker of susceptibility to VA in humans, was recently linked to a potential mechanism of arrhythmogenesis where membrane potentials of neighboring myocytes alternate with opposite phase (i.e. discordant alternans) forming the substrate for block and reentry. These findings suggest the intriguing hypothesis that alternans may represent a novel electrophysiological mechanism that transforms physiological into pathophysiological heterogeneities of repolarization which form the substrate for VA. Due to limitations of conventional electrophysiological methods there remain major gaps between our understanding of the spatial and functional distribution of ion channels at the cellular level, and the role these properties play in the kinetics of repolarization, alternans, and reentrant arrhythmias at the whole heart level. This proposal seeks to apply techniques of high-resolution optical mapping with voltage- sensitive dyes, a novel system for dual voltage-calcium imaging in the intact heart, and calcium imaging in voltage-clamped isolated myocytes, to experimental models where key pathophysiological components of VA (e.g. structural barriers, electrophysiological heterogeneities between cells, expression of cardiac gap junctions, left ventricular hypertrophy and failure) are carefully controlled. The aims of this project are to: 1. Determine the ionic bases for cellular alternans and discordant alternans between cells, with emphasis on cellular calcium cycling; 2. Determine how these ionic mechanisms are manifest at the tissue level in terms of mechanisms of discordant alternans, with particular emphasis on heterogeneities of restitution between cells and cardiac memory; 3. Determine the role of inter- cellular uncoupling caused by structural barriers or reduced gap junction expression in the mechanism of discordant alternans; 4. Establish discordant alternans as a singular mechanism for unidirectional block which underlies both monomorphic VT and VF; and 5. Examine the effects of pathophysiological changes in repolarization and structure on alternans-induced reentry in the hypertrophy and failing heart. These studies are expected to improve our understanding of the functional organization of electrical activity in the heart and provide important insights into the mechanisms, diagnosis, and possible treatment of life- threatening VA in humans.